The present invention relates to a catalyst for purifying exhaust gases from an internal combustion engine. In particular, it relates to a lean NOx catalyst.
It is well known in the art to use catalyst compositions, including those commonly referred to as three-way conversion catalysts (xe2x80x9cTWC catalystsxe2x80x9d) to treat the exhaust gases of internal combustion engines. Such catalysts, containing precious metals like platinum, palladium, and rhodium, have been found both to successfully promote the oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO) and to promote the reduction of nitrogen oxides (NOx) in exhaust gas, provided that the engine is operated around balanced stoichiometry for combustion (xe2x80x9ccombustion stoichiometryxe2x80x9d; i.e., between about 14.7 and 14.4 air/fuel (A/F) ratio).
However, fuel economy and global carbon dioxide (CO2) emissions have made it desirable to operate engines under lean-bum conditions, where the air-to-fuel ratio is somewhat greater than combustion stoichiometry to realize a benefit in fuel economy. Diesel and lean-burn gasoline engines generally operate under highly oxidizing conditions (i.e., using much more air than is necessary to burn the fuel), typically at air/fuel ratios greater than 14.7 and generally between 19 and 35. Under these highly lean conditions, typical three-way catalysts exhibit little activity toward NOx reduction, as their reduction activity is suppressed by the presence of excess oxygen.
The control of NOx emissions from vehicles is a worldwide environmental problem. Lean-burn, high air-to-fuel ratio, and diesel engines are certain to become more important in meeting the mandated fuel economy requirements of next-generation vehicles. Development of an effective and durable catalyst for controlling NOx emissions under net oxidizing conditions accordingly is critical.
Recently, copper-ion exchanged zeolite catalysts have been shown to be active for selective reduction of NOx by hydrocarbons in the presence of excess oxygen. Platinum-ion exchanged zeolite catalyst is also known to be active for NOx reduction by hydrocarbons under lean conditions. However, this catalytic activity is significant only in a narrow temperature range around the lightoff temperature of hydrocarbon oxidation. All the known lean-NOx catalysts reported in the literature tend to lose their catalytic activity for NOx reduction when the catalyst temperature reaches well above the lightoff temperature of hydrocarbon oxidation. This narrow temperature window of the lean-NOx catalysts is considered to be one of the major technical obstacles, because it makes practical application of these catalysts difficult for lean-burn gasoline or diesel engines. As an example, the Cu-zeolite catalysts deactivate irreversibly if a certain temperature is exceeded. Catalyst deactivation is accelerated by the presence of water vapor in the stream and water vapor suppresses the NO reduction activity even at lower temperatures. Also, sulfate formation at active catalyst sites and on catalyst support materials causes deactivation. Practical lean-NOx catalysts must overcome all three problems simultaneously before they can be considered for commercial use. In the case of sulfur poisoning, some gasoline can contain up to 1200 ppm of organo-sulfur compounds. Lean-NOx catalysts promote the conversion of such compounds to S02 and S03 during combustion. Such S02 will adsorb onto the precious metal sites at temperatures below 300xc2x0 C. and thereby inhibits the catalytic conversions of CO, CxHy(hydrocarbons) and NOx. At higher temperatures with an Al2O3 catalyst carrier, SO2 is converted to SO3 to form a large-volume, low-density material, Al2(SO4)3, that alters the catalyst surface area and leads to deactivation. In the prior art, the primary solution to this problem has been to use fuels with low sulfur contents.
Another alternative is to use catalysts that selectively reduce NOx in the presence of a co-reductant, e.g., selective catalytic reduction (SCR) using ammonia or urea as a co-reductant. Selective catalytic reduction is based on the reaction of NO with hydrocarbon species activated on the catalyst surface and the subsequent reduction of NOx to N2. More than fifty such SCR catalysts are conventionally known to exist. These include a wide assortment of catalysts, some containing base metals or precious metals that provide high activity. Unfortunately, just solving the problem of catalyst activity in an oxygen-rich environment is not enough for practical applications. Like most heterogeneous catalytic processes, the SCR process is susceptible to chemical and/or thermal deactivation. Many lean-NOx catalysts are too susceptible to high temperatures, water vapor and sulfur poisoning (from SOx).
Yet another viable alternative involves using co-existing hydrocarbons in the exhaust of mobile lean-burn gasoline engines as a co-reductant and is a more practical, cost-effective, and environmentally sound approach. The search for effective and durable non-selective catalytic reduction xe2x80x9cNSCRxe2x80x9d catalysts that work with hydrocarbon co-reductant in oxygen-rich environments is a high-priority issue in emissions control and the subject of intense investigations by automobile and catalyst companies, and universities, throughout the world.
A leading catalytic technology for removal of NOx from leanburn engine exhausts involves NOx storage reduction catalysis, commonly called the xe2x80x9clean-NOx trapxe2x80x9d. The lean-NOx trap technology can involve the catalytic oxidation of NO to NO2 by catalytic metal components effective for such oxidation, such as precious metals. However, in the lean NOx trap, the formation of NO2 is followed by the formation of a nitrate when the NO2 is adsorbed onto the catalyst surface. The NO2 is thus xe2x80x9ctrappedxe2x80x9d, i.e., stored, on the catalyst surface in the nitrate form and subsequently decomposed by periodically operating the system under stoiciometrically fuel-rich combustion conditions that effect a reduction of the released NOx (nitrate) to N2.
The lean-NOx-trap technology has been limited to use for low sulfur fuels because catalysts that are active for converting NO to NO2 are also active in converting SO2 to SO3. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SOx adsorbs more strongly on NO2 adsorption sites than NO2, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. Such presence of SO3 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Attempts with limited success to solve such a problem have encompassed the use of selective SOx adsorbents upstream of lean NOx trap adsorbents. Furthermore, catalytic oxidation of NO to NO2 is limited in its temperature range. Oxidation of NO to NO2 by a conventional Pt-based catalyst maximizes at about 250xc2x0 C. and loses its efficiency below about 100 degrees and above about 400 degrees. Thus, the search continues in the development of systems that improve lean NOx trap technology with respect to temperature and sulfur considerations.
Another NOx removal technique comprises a non-thermal plasma gas treatment of NO to produce NO2 which is then combined with catalytic storage reduction treatment, e.g., a lean NOx trap, to enhance NOx reduction in oxygen-rich vehicle engine exhausts. In the lean NOx trap, the NO2 from the plasma treatment is adsorbed on a nitrate-forming material, such as an alkali material, and stored as a nitrate. An engine controller periodically runs a brief fuel-rich condition to provide hydrocarbons for a reaction that decomposes the stored nitrate into benign products such as N2. By using a plasma, the lean NOx trap catalyst can be implemented with known NOx adsorbers, and the catalyst may contain less or essentially no precious metals, such as Pt, Pd and Rh, for reduction of the nitrate to N2. Accordingly, an advantage is that a method for NOx emission reduction is provided that is inexpensive and reliable. The plasma-assisted lean NOx trap can allow the life of precious metal lean NOx trap catalysts to be extended for relatively inexpensive compliance to NOx emission reduction laws. Furthermore, not only does the plasma-assisted lean NOx trap process improve the activity, durability, and temperature window of lean NOx trap catalysts, but it allows the combustion of fuels containing relatively high sulfur contents with a concomitant reduction of NOx, particularly in an oxygen-rich vehicular environment. What is needed in the art is an exhaust gas catalyst system having improved durability, as well as effective NOx management, over extended operating time. The present invention overcomes many of the shortcomings of the prior art.
A NOx catalyst structure typically includes 2 parts: a matrix support and an active catalyst component. The matrix is the backbone that allows the gasses to flow easily through the entire catalyst bed. The matrix generally consists of large particles with large pores; the active catalysts generally are much smaller particles and have much smaller pore sizes.
Diesel engines and engines that are lean burn usually operate in the range of 150xc2x0 C. to about 350xc2x0 C. Barium alumina typically has NOx to N2 conversions of xcx9c40% at 300xc2x0 C., xcx9c80% at 350xc2x0 C. and xcx9c40% at 400xc2x0 C. Barium zeolite typically has NOx to N2 conversion of xcx9c40% at 175xc2x0 C., xcx9c70 at 250xc2x0 C. and xcx9c40% at 350xc2x0 C. As a vehicle warms to 150xc2x0 C., 100% of the N2 is formed on the barium zeolite. As the temperature increases to 250xc2x0 C., 60% of the N2 formed is on barium zeolite and 40% is on barium-alumina. As the temperature increases further to 350xc2x0 C., 80% of the N2 is formed on the barium-alumina and only 20% of the N2 is formed on the barium-zeolite.
Now, according to the present invention, a lean NOx catalyst is provided for use in a non-thermal plasma exhaust gas treatment system. The presently invented catalyst comprises a combination of an alkaline earth-zeolite catalyst with an alkaline earth-alumina catalyst.
The alumina catalyst preferably comprises coarse aluminum oxide particles are having an average size ranging from about 10 to about 30 microns. The zeolite catalyst preferably comprises zeolite particles having an average size ranging from about 0.1 to about 0.3 microns. In an admixture of the alumina catalyst and the zeolite catalyst, the fine zeolites tend to fill in the void spaces around the coarse alumina particles. The small zeolites particles are densely packed. Exhaust does not flow easily through the dense packed zeolites. The aluminum oxide particles are mostly porous. Gasses can easily pass through the aluminum oxide particles to reach the zeolite particles.
The aluminum oxide particles providing the admixture matrix generally comprise agglomerations of small psuedocrystalline alumina particles typically of about 0.3 microns or less. The agglomerations preferably are larger than about 10 microns and less than about 30 microns. The alumina matrix is mixed with a Ba-zeolite catalyst component, typically featuring a particulate size ranging from about 0.1 to about 0.3 microns. In general, the smaller the zeolite particle, the more hydrothermally stable the zeolite becomes, thereby improving its long term activity. Preferably, pursuant to the invention, the zeolite active catalysts are dispersed throughout the alumina matrix.
When the catalyst structure is disposed as an exhaust gas treatment catalyst, large gaseous molecular materials are enabled to enter the large pores of the alumina matrix without plugging the small pores of the zeolite catalyst. The large gaseous molecular materials are decomposed through action of the alumina matrix to small fractions, whereby the small fractions then are able to enter the micropores of the active zeolite catalysts. Large heteroatoms, such as sulfur and nitrogen, remain are adsorbed onto and remain on the reactive alumina matrix surface, such that the more active zeolite is not neutralized or poisoned by these heteroatoms and accordingly remains highly active. The matrix of alumina further is especially reactive to and can trap deposited metals such as nickel. Poisons will deposit on the first high surface area material that they are exposed to. SEM of engine aged catalysts show poison deposition throughout the large aluminum oxide particles. The zeolite particles show little poison deposition. This demonstrates that exhaust flows through the aluminum oxide before reaching the zeolite.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, and appended claims.
The catalyst of the present invention preferably comprises a barium alumina fraction in a proportion ranging from about 30 wt % to about 50 wt %, and a barium zeolite fraction in a proportion ranging from about 50 wt % to about 70 wt %. A particularly preferred admixture comprises a barium alumina fraction in a proportion ranging from about 35 wt % to about 45 wt %, and a barium zeolite fraction in a proportion ranging from about 55 wt % to about 65 wt %. Inclusion of an alkaline binder in the admixture is preferred to prevent agglomerated alumina particles being broken down into primary aggregates.
As described above, the aluminum oxide particles generally are agglomerates of about 0.3 micron or less aluminum oxide grains. If low pH aluminum compounds are used as binders, the aluminum oxide agglomerates tend to be broken down to the primary particles. If that happens, for example, a mixture of about 0.3 micron aluminum oxide and about 0.3 micron zeolite would result. The washcoat from such a mixture would be densely packed. This type of washcoat would be easily poisoned and diffusion limited at the surface.
Preferably, high pH aluminum compounds are used as binders. Using such binders, the aluminum oxide agglomerates appear stable and maintain their integrity as about 10 to 30 micron particles. A mixture, for example, of such 10-30 micron alumina particles and 0.3 micron particles provides a preferred washcoat. Accordingly, a binder comprising an alkaline aluminum, such as a barium aluminum hydroxide or an ammonium aluminum hydroxide is preferred. An ammonium aluminum hydroxide is particularly preferred. When calcined, it is preferred that the ammonium aluminum hydroxide should provide at least about 2 wt % of the washcoat mass. Inclusion of an ammonium aluminum hydroxide binder that provides at least about 4 wt % washcoat mass is more preferred, and about 6 wt % washcoat mass is particularly preferred.
Any alkaline earth element may be used as the active catalyst. For example, the active catalyst element may comprise calcium, strontium, and/or barium. The use of barium is particularly preferred. Tests have indicated that calcium-doped catalysts generally convert about 30% NOx to N2; strontium-doped catalysts generally convert about 50% NOx to N2; barium-doped catalysts generally convert about 70% NOx to N2. Based on a desire in the industry to optimize conversion of NOx to N2 at about 90% or better, barium is the particularly preferred occluding catalyst material.
Accordingly, Ba is the preferred occluding catalyst for both the zeolite and the alumina components of the present catalyst. The alumina matrix features enhanced trapping efficiency if it is doped with a material such as barium. In specific, nitrogen species can neutralize catalytically active sites. Barium is the most robust alkaline earth element for resistance to nitrogen poisoning. The alumina matrix provides sacrificial sites for nitrogen poisoning, thus precluding large polycyclic nitrogen compounds from entering and poisoning the small pores of the zeolite catalyst component.
The zeolite catalyst component preferably comprises a barium content of about 18 wt % to about 36 wt %; about 24 wt % to about 30 wt % is particularly preferred. Any type zeolite may be used; preferred zeolites include X type zeolite, Y type zeolite, and/or ZSM-5 type zeolite. A Y type zeolite is particularly preferred. A zeolite surface area of at least about 300 m2/gram is preferred, at least about 400 m2/g is more preferred, and a surface area of at least about 500 m2/g is particularly preferred. The preferred zeolite average particle size is less than about 0.9 microns; more preferred are zeolite particles of an average size less than about 0.6 microns; and particularly preferred are zeolites having an average particle size less than about 0.3 microns. The zeolite particles preferably feature average pore sizes ranging from about 4 to about 10 angstroms (xe2x80x9cAxe2x80x9d), with average pore sizes ranging from about 7 to 8. A particularly preferred. It is preferred to stabilize the zeolite catalyst with the inclusion of a rare earth element. Inclusion of a lanthanum oxide stabilizer is particularly preferred. Preferably, the zeolite has a silica to alumina ratio of at least about 2; a ratio of at least about 4 is more preferred; and, a ratio of at least about 7 is particularly preferred.
A ZSM-5 zeolite with pores of about 4 to 5 A is preferred and a lanthanum stabilized X zeolite is more preferred and a Y-type zeolite with pores of about 7 to 8 A is greatly preferred and a rare earth stabilized Y-type zeolite is especially preferred. Lanthanum oxide is the rare earth stabilizer of choice.
The alumina matrix catalyst component preferably has a barium content of at least about 14 wt %; at least about 21 wt % is more preferred; and, at least about 28 wt % is particularly preferred. Exhaust deposits of oil derived xe2x80x9cglassyxe2x80x9d compounds such as calcium phosphate and zinc phosphate can greatly reduce diffusion. High levels of alkaline earths, such as barium, prevent formation of these diffusion limiting barriers.
Preferably, the alumina has a surface area of at least about 150 m2/gram; a surface area of at least about 200 m2/g is more preferred; and, a surface area of at least about 250 m2/g is particularly preferred. An average alumina pore size of at least about 40 A is preferred; a pore size of at least about 60 A is more preferred; and, a pore size of at least about 80 A is particularly preferred. Specific acidity ranging from about 50 mmoles (millimoles) n-butylamine/m2xc3x9710xe2x88x924 to about 500 mmoles n-butylamine/m2xc3x9710xe2x88x924 is preferred; a specific acidity of about 350 mmoles n-butlyamine/m2xc3x9710xe2x88x924 is particularly preferred.
Suitable barium sources for preparation of a barium-alumina component include barium nitrate, barium acetate, barium hydroxide, barium ethoxide, barium isopropoxide, and/or barium 2-ethylhexanoate. Barium acetate, barium isopropoxide, and barium 2-ethylhexanoate are preferred. Barium 2-ethylhexanoate is particularly preferred.
Suitable aluminum sources are aluminum oxide, aluminum hydroxide (AIOOH) boehmite and pseudoboehmite, aluminum methoxide, aluminum n-butoxide, aluminum ethoxide, and/or aluminum isopropoxide. Aluminum isopropoxide, aluminum oxide, and aluminum hydroxide are preferred. Aluminum hydroxide is particularly preferred.
The surface acidity of the alumina may be increased by the addition of silica. Suitable silica stabilized aluminas include Condea Vista""s SIRAL 5 with 5 wt % silica, SIRAL 10 with 10 wt % silica, SIRAL 20 with 20 wt % silica, SIRAL 30 with 30 wt % silica, and/or SIRAL 40 with 40 wt % silica. SIRAL 30, SIRAL 20, and SIRAL 10 are preferred. SIRAL 10 is particularly preferred.
The alumina preferably includes additional doping ions, such as M+3 cations. Lanthanum, yttrium, and/or scandium are the preferred dopants; scandium is particularly preferred. A doping level of less than about 3 wt % is preferred; less than about 2 wt % is more preferred; and, about 1 wt % is particularly preferred. Suitable doping reagents include lanthanum 2-ethylhexanoate, yttrium 2-ethylhexanoate, and scandium 2ethylhexanoate.